7
6.5 6
5.5 5
4.5 4
3.5 3
0 2 4 Month
Figure 3: The EER value trend of a chiller without free cooling when operating with a fixed water outlet temperature
6 8 December EER January November March October July
8
7.5 7
6.5 6
5.5 5
Outlet temperature
4.5 4
0 5 10 15 Time
Figure 4: The outlet water temperature trend. Inverter control achieves much greater water accuracy than is possible through control with a standard screw compressor
20 25 30
screw compressors can attain COP> 8 and EER > 7. Figure 3 shows the EER value trend of a
chiller without free cooling when operating with a water outlet temperature of 5C. This example chiller is designed for 680 kW at full load. During the warmer months of the season (June-August) the required load is 100%, whilst during the colder period (November-March), when ambient temperature is much lower, the chiller load is estimated between 50% and 60%. In the hotter months, during the most
demanding ambient conditions, EER has a minimum value of 4, whilst when the chiller load is around 60%, the EER values are typically between 6 and 7, much higher than a standard chiller. All these values relate to the maximum ambient temperature for each month, so these EER are considered a minimum. Figure 4 shows the outlet water temperature trend. Inverter control achieves much closer water temperature control than is possible with a standard screw compressor. In this example the water temperature fluctuation is only +/- 0.5C. The set point value is established more rapidly than might be expected using traditional chiller control. The ESEER is the EU index used to
classify chiller performance (and differs slightly from the UK SEER that typically takes more account of peak load operation); and for chillers with inverter control led screw compressor (and variable volume index, Vi), the increase for this example ESEER is around 15%.
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Inverter controlled air cooled condenser fans Inverters are also applicable to axial fans, and so can control the air circulated through the condensers in air cooled chillers in place of simple on/off or step control to steadily reduce the air flow and achieve improved condenser control. The benefits will not only be in improving the efficiency ratios by around 10%, but also by reducing noise levels for significant periods of time when the system is running at part load (typically up to 6dBA).
Conclusions There are many opportunities to apply innovative solutions to reduce the absorbed power requirements of refrigeration hardware in commercial air conditioning and data centre facilities. The optimum operation of refrigeration equipment at part loads is especially significant in conditions where the average annual ambient air temperatures are between 5C and 20C – typical of the vast majority of European conditions. For even lower ambient temperatures, the combination of inverter technology, coupled with that of free cooling – whereby chilled water can be produced using only fans’ energy – can be effectively used to produce chiller units with even greater efficiencies than previously considered possible. CJ
l Maurizio BurBa is managing director of Hitema.
www.hitema.it A version of this article first appeared in the October 2011 edition of REHVA Journal.
www.rehvajournal.com
Inverter control achieves much closer water temperature control than is possible with a standard screw compressor
February 2012 CIBSE Journal 45
EER
Temperature ( C)
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